1. Field of the Invention
The invention relates to a powder based on cerium oxide, a process for preparing the powder, and uses of the powder. The invention further relates to a dispersion containing the powder.
2. Description of the Related Art
Cerium oxide is a significant constituent of catalysts. Furthermore, cerium oxide is an important material for the polishing of glass and electronic components. Particularly in the field of chemomechanical polishing, the journal and patent literature describes numerous ways for improving the properties of cerium oxide powders and of cerium oxide dispersions.
Cerium oxide is usually prepared by calcination of cerium hydroxides or cerium carbonates. The calcined oxides are then milled and sieved to form powders.
Wet-chemical routes and gas-phase processes for preparing cerium oxide are also known. Cerium oxide can be prepared by hydrothermal synthesis, as described, for example, in U.S. Pat. No. 5,389,352. Here, cerium(III) salts are converted oxidatively at elevated temperature and pressure into cerium oxide which crystallizes out in the form of fine particles.
Gas-phase processes such as spray pyrolysis methods in which a cerium oxide precursor, usually in the form of an aerosol, is oxidized in a flame are of particular interest. One such method is described in U.S. published application 2001036437.
In J. Mater. Res., Vol. 17, pages 1356-1362 (2002), Pratsinis et al. describe the preparation of cerium oxide powders of high crystallinity by flame spray pyrolysis of cerium acetate solutions in solvent mixtures comprising acetic acid, isooctane and 2-butanol by oxidation by means of oxygen in an oxygen/methane flame. The solvent mixture is essential to the preparation of a cerium oxide powder which is free of relatively coarse particles.
U.S. published application 2004126298 describes a process for preparing metal oxides, in which droplets are produced from a solution and these are oxidized in a flame. The solution contains at least one starting material and at least 60%, based on the total solution, of a carboxylic acid as solvent.
A process for preparing cerium oxide comprising coarse and fine material is known from U.S. Pat. No. 6,887,566. This process discloses the combustion of an aerosol of a cerium oxide precursor in a high-temperature zone of 700-1,100 K.
DE-A-10251029 discloses a pyrogenic cerium oxide powder having a coarse fraction comprising crystalline, nonaggregated particles having a mean diameter of from 30 to 200 nm and a fine fraction in the form of aggregates of finely crystalline, grown-together primary particles having a mean aggregate diameter of from 5 to 50 nm and a BET surface area of from 15 to 200 m2/g.
U.S. published application 2005036928 discloses a polycrystalline cerium oxide powder in the form of aggregates of primary particles having a specific surface area of from 70 to 150 m2/g, a mean primary particle diameter of from 5 to 20 nm and a mean, projected aggregate diameter of from 20 to 100 nm and a dispersion containing this cerium oxide powder. Furthermore, numerous dispersions containing cerium oxide for chemomechanical polishing are described.
U.S. Pat. No. 6,663,683 discloses an aqueous dispersion containing pyrogenic silicon dioxide doped with cerium oxide, with the cerium oxide being introduced via an aerosol of a cerium salt solution or suspension and the mean particle size in the dispersion being less than 100 nm.
U.S. Pat. No. 6,767,377 discloses an aqueous dispersion which contains particles having a core of doped, pyrogenic low-structured silicon dioxide and a cerium oxide shell and does not exceed a mean secondary particle size of 0.2 μm.
U.S. published application 2005074610 discloses a dispersion of pyrogenic cerium oxide which does not have any particles larger than 1 μm.
U.S. Pat. No. 6,827,752 describes a cerium oxide dispersion which is stabilized by polyacrylates.
U.S. Pat. No. 6,443,811 describes a cerium oxide dispersion in a neutral to weakly basic medium which contains a cationic surface-active agent.
U.S. Pat. No. 6,420,269 describes a cerium oxide dispersion which has a pH in the range from 8 to 12 and contains a dispersant, preferably a water-soluble organic polymer, a water-soluble anionic or nonionic surface-active agent or a water-soluble amine.
A disadvantage of the cerium oxide dispersions according to the prior art is that satisfactory stability in the neutral to weakly alkaline range can be achieved only in the presence of additives.